The present invention is a float-zone process for forming particulate silicon into monocrystalline silicon. The process employs a reusable silicon conduit having a lower end heated to form a melt zone and an upper end provided with a means for delivering a controlled amount of particulate silicon to the melt zone. A monocrystal silicon seed is used to pull a monocrystalline silicon ingot from the melt zone and particulate silicon is added to the melt zone to replenish silicon removed as monocrystalline silicon.
There are two commonly employed methods for producing semiconductor-grade monocrystalline silicon. One method. known as the Czochralski method, employs a silicon melt contained in a suitable crucible from which a monocrystalline silicon ingot is pulled. The purity of monocrystalline silicon prepared by this method is compromised by impurities occurring as a result of deterioration of the crucible wall and leaching from the crucible wall. The second common method for producing semiconductor-grade monocrystalline silicon is the float-zone process. In this process, a rod of polycrystalline silicon is converted to monocrystalline silicon by passing a molten zone along the length of the rod. This process avoids the crucible contamination problem associated with the Czochralski process, but is limited by the ability to produce polycrystalline rods that are crack-free and of the desired size.
A crucible-free process for float-zone processing particulate silicon into a monocrystalline element can overcome the cited limitations of the Czochralski process and the float-zone process using solid polycrystalline silicon rods. Therefore, semiconductor-grade monocrystalline silicon of greater purity can be produced, than is possible by the Czochralski process, and the expense and size limitations of polycrystalline silicon rods can be avoided.
British patent No 1,081,827, Published Sep. 6, 1967, describes the basic float zone process for processing a polycrystalline silicon rod into a monocrystalline silicon rod. A small portion of the length of the polycrystalline silicon rod is heated so that a melt zone is formed and by relative movement between the heating source and the rod, the molten zone is passed through the rod, from one end to the other. If a monocrystal silicon, seed is attached to one end of a rod, the whole rod starting from this seed crystal can be converted into a single crystal.
Rummel, U.S. Pat. No. 2,907,642, issued Oct. 6, 1959, describes a process where powdered silicon is blown into a melt zone to be incorporated therein and contribute to the formation of a monocrystalline silicon rod.
Emeis, U.S. Pat. No. 2,930,098, issued Mar. 29, 1960, proposed preparing particulate samples for possible float-zone treatment by sintering the particles in a quartz tube. The lower temperature required for sintering is reported to reduce contamination of the silicon due to deterioration of the quartz tube.
Imber, U.S. Pat. No. 3,156,533, issued Nov. 10, 1964, describes a process for converting powdered silicon into monocrystalline silicon with minimal contamination. The described invention relates to an apparatus for growing crystals which provides a conical crucible supported within a cylindrical chamber, and a powder dispensing means, or hopper suspended in the upper part of the chamber directly above the conically shaped crucible. A discharge opening in the apex of the conically shaped crucible communicates with the lower part of the chamber where a seed crystal is supported directly beneath the opening. In operating the device, powder spills from the hopper onto the hot conical crucible where the powder melts and collects in a molten pool over the discharge opening of the cone. Drops of molten silicon form and fall through the opening to the crystal grown below.
It is an objective of the instant process to provide a crucible-free process for converting polycrystalline silicon particles into monocrystalline silicon, thereby avoiding the potential contamination associated with the use of a crucible to contain a silicon melt. A second object is to provide a process that avoids the difficult and expensive process of preparing polycrystalline silicon elements for float-zone processing. A third objective is to provide a process where silicon particles can be directly processed by a float-zone process without sintering or other processing to create a formed element. A fourth objective is to provide a process where the section of the chamber of the zoning apparatus used for feed material can be reduced, thereby providing the potential to grow longer monocrystalline silicon ingots in a size restrained facility. A fifth objective is to provide a process were the monocrystalline silicon ingot is not limited in size by the availability of polycrystalline silicon rods suitable for float-zone processing.